1. Field of the Invention
There are many nondestructive inspection methods presently being applied to tires. These use X-radiography, infrared, holography and ultrasonic methods.
This invention relates to the testing of tires to determine their condition using nondestructive testing by a reflection pulse ultrasonic system.
2. Description of the Prior Art
Various pulse ultrasonic systems have been developed for inspection of tires. Known U.S. patents in this field are U.S. Pat. Nos. 2,378,237; 3,148,535 and 3,815,407.
U.S. Pat. No. 2,378,237 discloses a method and apparatus for ultrasonic testing of tires to locate separations or breaks in a tire carcass. In the system of that patent, ultrasonic waves are directed to strike a portion of a tire immersed in liquid. Resultant sound waves from another portion of the tire are picked up. This other portion is generally opposed to that portion of the tire receiving the sound waves. That other portion of the tire may be another peripheral portion or may be the inner portion in alignment with the outer surface to which the sound waves are applied. The picked up ultrasonic waves are changed by means to provide a sensuous indication using, for example, an oscillograph, a voltmeter, a rectifier and D.C. ammeter, or an oscillator. Relative movement between the tire and the ultrasonic waves imparted to the tire is effectuated to subject different portions of the tire to the ultrasonic energy. Any change in the sensuous indication that is noted during this relative movement indicates a separation or break in the tire because the change occurs as the ultrasonic waves are reflected or otherwise dispersed from a separation or break so that the ultrasonic wave is not transmitted to the indicator and thus is not registered by the indicator. The reflected ultrasonic energy is not received by the indicator. In use, the system of this patent provides an adjustment of the indicator so that there is a full scale deflection or trace of the indicator for a good portion of the tire being tested. An indicator is practically at a zero indication when during the relative movement between the tire and the transducer the portion of the tire containing a separation or break is under inspection. The system apparently provides this indication if the break is present at a portion or discrete portions about its periphery that can be compared with a portion that constitutes a good portion of the tire, that is, a portion that does not have a break or separation.
U.S. Pat. No. 3,148,535 discloses apparatus and a method for the inspection of tires using ultrasonic pulses to locate flaws and density changes caused by various foreign bodies, such as nails, in the tire, changes in the shape of the tire, etc. In the apparatus of that patent the ultrasonic transducer is used to impart ultrasonic energy to a wall portion of the tire and is used also to receive reflected ultrasonic energy from the wall portion of the tire so that the system is in this case a pulse-echo ultrasonic testing system. In a modification of the apparatus another transducer is mounted to scan in a radial direction for inspection of the rubber material on the periphery of the tire. The entire reflected ultrasonic energy is converted into electrical signals that are used to modulate a visual display means, such as a cathode ray tube, to present visible images or traces on the cathode ray tube.
The apparatus of this patent included means for rotating the tire by rotating the wheel on which the tire remains mounted. During this rotation the apparatus continuously scans a predetermined cross section of the tire using the pulse-echo ultrasonic testing system described above. Thereby a predetermined annular area of the tire is totally scanned during a complete rotation of the tire. The operation of the equipment, because of its construction, will provide a changing image pattern on the face of an image scope which pattern may be visually analyzed to determine the extent of the flaws or location of foreign bodies such as nails in the rubber of the tires.
Alternatively in the method of U.S. Pat. No. 3,148,535 a predetermined area of the wall of the tire also can be automatically scanned using the ultrasonic testing system with movement of the pulse-echo transducer and following the scanning of that area the wheel is turned so that a new area of the tire is in the scanning field. After each scanning of an area there is this intermittent turning of the wheel until the entire annular area of the wall of the tire has been scanned during one rotation of the wheel and tire. Apparently during these scans the transducer mounted for an inspection of the rubber material on the periphery of the tire is also operated to inspect the entire periphery, presumably of the width of tread, with an analysis of the trace from that scan to detect flaws and density changes. This alternative method, because of the construction of the equipment, also uses a changing image pattern that occurs around the tire if there are flaws or foreign bodies at one or more particular locations in the tire.
U.S. Pat. No. 3,815,407 discloses apparatus and method for tire inspection using ultrasonics. In the system the apparatus has a source transducer, i.e., a transmitter transducer, mounted to provide ultrasonic energy to the crown area of the tire, apparently at the center line of the tread, and has opposed receiver transducers mounted to receive ultrasonic energy from oppositely disposed points of the sidewalls of the tire being tested. The circuitry connected to the three transducers provides means to measure an imbalance in the transmission characteristics of the tire from the tread area to the oppositely disposed points of the sidewalls. The information collected permits identification of both the presence and nature of hidden defects. The tire is rotated during the testing procedure. Cuts, tread chunking, uneven wear, broken cords and ply separations will in general upset the symmetry of the tire. The defect detection of the invention of the patent is based on observing changes in symmetry and resonant frequency. The apparatus has means to control the frequency of the source transducer and during the rotation of the tire the frequency imparted by the source transducer to the tire is varied to compensate for changes in resonant frequency caused by physical variations in the tire. At the same time a lack of symmetry is detected in the transmission characteristics in the region of the casing by the pair of receiver transducers. The patent states that a ply separation will cause minimal change in resonant frequency but will change the damping and this change in damping with the defect off the plane of symmetry will unbalance the output of the two receiver transducers. When the separation or other defect in the tire is on the center line this will cause a change in the magnitude of the signals provided by both receiver transducers and because of the presence of the differential amplifier in the circuitry there is provided an indication of a symmetrical defect. The system of U.S. Pat. No. 3,815,407 is not a reflection ultrasonic system.
Australian investigators, using retreaded aircraft tires, have found that low-frequency, thru-transmission ultrasonic attenuation generally increases with tire age (i.e., number of times the tire is retreaded). The Australian airlines who use thru-transmission ultrasonic investigation to check retreading around the entire periphery of the tire have adopted a rule of rejecting any tire whose attenuation of signal is sufficiently great that defect observation is questionable. As a result, blown retreaded tires have been almost eliminated from the operation of the Australian airlines. This thru-transmission ultrasonic system for examining retreaded tires cannot discriminate between retread bond line anomalies and degraded plies of the carcass. Thus such system does not determine residual carcass life, unless the tire is buffed.
High-frequency ultrasonic instruments for nondestructive testing and thickness gauging of materials are commercially available. The test instrument utilizes transistorized circuits of printed circuit board construction and a cathode-ray tube (CRT) for video display of the test data. The instrument, that is capable of using either direct contact or immersion testing techniques and that can be used for single or dual transducer testing, includes: a power supply; a clock; a main bang generator; a sweep circuit; a receiver circuit; and a display circuit. The power upply provides the operating power to all the circuits in the tester. It also provides synchronization of triggering signals initiated by the clock. The clock triggering signals trigger the main bang generator and the sweep circuit simultaneously. When the main bang generator is triggered, it sends a pulse of electrical energy to the transducer and the receiver circuit. This pulse of electrical energy is called the main bang. The transducer converts the main bang into sound vibrations and transmits these sound vibrations into the object under test. The sweep circuit is triggered at the same instant the main bang generator is triggered. The sweep circuit provides the horizontal sweep voltage for the sweep trace which appears on the CRT. The horizontal sweep trace appears on the CRT shortly before the main bang because a built-in delay allows the sweep to start before the main bang. The received signals reflected by the boundaries from within the object under test are received by the transducer and applied to the receiver circuit. The receiver circuit amplifies and detects the received signals and converts these signals into video. The video signals from the receiver are applied to the display circuit and appear on the CRT. The display circuit permits the main bang, horizontal sweep and the received signals to be displayed on the CRT. The entire process is typically repeated 200 to 3000 times per second, depending upon the selected clock pulse frequency.